Formation and evolution of the coke precursors on the zeolite catalyst in the conversion of methanol to olefins

Resolving the deactivation mechanism of zeolite in methanol to olefins (MTO) and clarifying the formation and evolution of coke precursors (e.g., polycyclic aromatic hydrocarbons [PAHs]) are crucial to understanding the whole MTO process; however, they are challenging due to the extraordinarily complex reaction network. Herein, the deactivation behavior of typical zeolite catalysts (viz., H-SSZ-13, H-beta, and H-ZSM-5) in MTO was investigated. The results indicate that the alkylation of cyclic intermediates (e.g., methylbenzene and cyclohexene) with cyclic carbocations (e.g., cyclopentadienyl and cyclohexadienyl cations) is a main manner to form PAHs and that the cross-linked PAHs act as the primary coke precursors. In addition, the formation of coke precursors is related to zeolite pore structure. The large intra-crystalline space of H-SSZ-13 and H-Beta provides enough reaction space for the alkylation and cross-linking of cyclic intermediates, whereas for H-ZSM-5, with smaller intra-crystalline space, the alkylation and cross-linking of cyclic intermediates are more energy demanding.